In recent years, hybrid vehicles that improve fuel efficiency of an engine and reduce exhaust gas by using an engine in combination with a rotating electrical machine as a source for driving force have been put into practical use. Japanese Patent Application Publication No. 2008-120139 (JP-A-2008-120139), for example, describes one example of such a drive system used in a hybrid vehicle. The drive system described in JP-A-2008-120139 is a so-called split hybrid drive system that distributes driving force from the engine to an output member side and a first rotating electrical machine side using a power split device. As shown in FIGS. 1 and 2 of JP-A-2008-120139, in this hybrid drive system, an engine 1, a first rotating electrical machine 2, and an output member 5 are connected to rotating elements, respectively, of a power split device 4 that performs a differential operation using at least three rotating elements. Also, the output member 5 is connected to the output side of a speed change mechanism 6 that is capable of setting at least two speed ratios, i.e., hi and low, and a second rotating electrical machine 3 is connected to the input side of the speed change mechanism 6. Further, a clutch mechanism 8 is provided that selectively connects the engine 1 to the second rotating electrical machine 3. As a result, this hybrid drive system achieves various operating modes and have superb power transmission efficiency.
With the hybrid drive system described in JP-A-2008-120139, as the vehicle speed increases, the operating mode switches from a mode in which the clutch mechanism 8 is released and the power distributed from the engine 1 and the decelerated power from the second rotating electrical machine 3 are both directly transmitted to the output member 5, to a mechanical lock-up mode which is a parallel mode, and then to an output split mode in a so-called overdrive state in which the rotation speed of the output member is increased by the engine rotation, in that order. With this hybrid drive system structure, power circulation when the second electrical rotating machine 3 is used to generate power and the first rotating electrical machine 2 is used for powering in the output split mode can be suppressed by providing the clutch mechanism 8.
As is described in paragraph [0062] of JP-A-2008-120139, this hybrid drive system is structured such that when switching to the output split mode, the speed change mechanism 6 is first placed in a neutral state with the rotation speed of the first motor-generator 2 being zero or a low rotation speed close to zero. Then when the second motor-generator 3 and the engine are rotating in synchronization, the clutch mechanism 8 is engaged, thus completing the switch to the output split mode.
That is, in the system described in JP-A-2008-120139, when an attempt is made to switch the second electrical rotating machine between the output member side and the input rotating element side (i.e., the engine side) of the power split device, it is necessary to first place the speed change mechanism provided between the second electrical rotating machine and the output member in a neutral state, then synchronize the second electrical rotating machine with the input rotating element of the power split device, and then engage the clutch mechanism. However, this operation may result in shift shock being generated during the switch or a drop in driving force, and thus an unpleasant sensation may be imparted to an occupant.